Thermal fluid heaters have been used in the past when it has been desired to have an indirect heat source for downstream heat requirements and when for various reasons it has been undesired to use steam under pressure as the heat transfer medium. As opposed to traditional steam boilers which use water/steam under pressure as a heat transfer medium, thermal fluid heaters utilize a thermal fluid such as an inert synthetic oil which has a relatively high boiling point (such as 650.degree. F.) to provide a heat supply source and system that may be operated essentially at atmospheric pressure and which avoids the corrosive side effects of water/steam as a heat transfer medium. Conventional thermal fluid heaters, however, have had their own shortcomings, especially in the form of possessing less than optimum thermal heat transfer efficiencies.
One particular aspect of thermal fluid heaters that has led to sub-optimum heat transfer efficiency has been that the thermal fluids used therein have particular maximum temperatures (depending on the particular fluid used) at which point, if they are heated beyond, they begin to decompose and break down into heavier hydrocarbon substances. This can eventually lead to the fouling of the fluid transfer apparatus and the rendering of the entire system inoperative until time consuming and costly purging is performed. This has meant that extreme care has had to be taken in the design, construction and operation of thermal fluid heaters to ensure, even by application of extremely conservative safety factors, that the maximum operating temperature of the thermal fluid would not be exceeded at any point in the heater.
The typical thermal fluid heater is usually cylindrical in shape with a gas or oil fired jet burner mounted on one end thereon and with a cylindrical helically wound pipe coil interior of and running the length of the heater. The burner is typically fired down the center of the heater with the combustion flames and gases heating the surrounding helically wound coil and also the thermal fluid contained in the coil and being pumped therethrough. However, the aforementioned susceptibility of the thermal fluid to breakdown has complicated the design and operation of such heaters in that there are certain areas or "hot spots" in such heaters at which the temperature of the helical coil, and the fluid therein, has proved to be greater than that at other points; indeed, one particularly hot area has been found to be at the terminal end of the coil furthest from the combustion burner at which area it has been found that the flame from the burner has tended to rise and spread and even contact portions of the coil. Attempts made to avoid overheating of the thermal fluid have not truly recognized the cause of the problem and have led to even further shortcomings. For example, to avoid overheating it has been suggested to operate the burner at reduced heat generating capacity, or to construct the heater and heater coil of an enlarged and non-optimum size, to avoid flame contact anywhere on the coil. Yet all of such attempts have only resulted in heater operation with substantially less than overall optimum thermal efficiency and heater designs in which the true problem present was not faced or solved.
Efforts were made to increase the resulting reduced thermal efficiency of such heaters but these likewise did not meet the basic overheating problem. Multiple helical coils were provided in order to have increased heat transfer area in a given sized heater package. Alternatively, substantial insulating and refractory material were provided in the interior of such heaters to retain and reflect inwardly the heat generated by the combustion burner, but such material has been found to be highly susceptible to material failure in sustained heater operation so as to create substantial repair and replacement problems. Even preheating of combustion air, just prior to its being introduced to the combustion burner, has been suggested in an attempt to improve heater efficiency, but as stated none of these efforts have truly recognized or solved the true problem.